Thermionic device



March 12, 1929. E. Y. ROBINSON THERMIONIC DEVICE Filed July 24, 1923 3 Sheets-Sheet l C0//20L/e l arch 12, 1929. E. Y. ROBINSON 3,705,377

THERMIONIC DEV ICE Filed July 24, 1925 s Sheets-Sheet 5 I IIIIIIJIIIIIIJI... ,,",l,"II- 5 4 47 IIIIIIIIIIll1.---

IIIIIIlIIlIII Patented Mar. 12, 1929.

UNITED STATES I 1,705,311 PATENT OFFICE.

ERNEST YEOMAN IRbBINSON, OF MANCHESTER, ENGLAND, ASSIGNOR TO METROPOLI- TAN-VICKERS ELECTRICAL COMPANY LIMITED,

JQINT-STOCK COMPANY.

Application electric current theretlirough. Cathodes of this type are disadvantageous in several respects. For instance a large heating current is necessary when there are large space currents in the valves because the space currents add to the heating currents at the negative ends of the filaments and subtract from the heating currents at the positive ends of the filaments and this results in a non-uniform heating of the filaments. Consequently it is necessary to arrange that the heating current is many times the space current in order to reduce this non-uniform heating to a minimum. Moreover, filaments or wires are unsuitable for use in valve tubes wherein the voltage is high or the interelectrode gap is small owing to the diffi culties attendant upon rigidly supporting such filaments. To overcome these defects it has been proposed to construct the oathode of a sheet of material heated by a thermionic bombardment from an auxiliary filamentary cathode which is rendered incandescent by passing a current theret-hrough. It has also been proposed to construct the cathode of sheet material, said sheet hermetically dividing the valve into two compartments one of which contains the anode and the other of which contains means, for heating the sheet by a gaseous discharge on one face thereof.

In vacuum electric tubes constructed according to the present invention the cathode is constructed of two-or more portions separated by gaps which are preferably small and the operative temperature of the cathode is maintained by creating alternating or continuous potential differences between the cathode portions. As a result a thermionic current flows across the space between one cathode portion and the other, and the cathode is heated and 'maintained at its operative temperature by said thermionic OF LONDON, ENGLAND, A BRITISH 'rnamnromc Dav es.

filed July 24 1923, Serial No. 653,544, and in Great Britain August 8, 1922.

space current and by radiation of heat from one portion to another. The cathode may be brought up to the operative temperature at which it can maintain itself in various ways! For instance one portion may be preliminarily heated by passing a current. therethrough. In this case such portion is preferably so constituted that it oflers considerable resistance to the current which is employed to bring it up to the operative temperature at which the cathode can maintain itself. 'It ma for instance be con.- structed of a coil fi ament. The cathode or one of the cathode portions may be heated initially by a thermionic bombardment from .an auxiliary member to'which it serves in the relation of anode and the auxiliary member may consist of a heated filament. Or the cathode or a cathode portion may be heated by radiation from a heated filament. WVhen the vacuum tube device is continuously evacuated during operation by means of a vacuum pump the pressure of gas in the device may be increased and the electrodes initially heated by a gaseous discharge. This discharge may take place between the cathode portions, or between the cathode or a portion thereof and another member, such for instance as the anode.

When the cathode is heated and its temperature maintained according to this invention any portion thereof is heated by heat radiated from the other portion or portions and at such times that it is positive to any other portion of the cathode which is heated by a thermionic bombardment. For instance where the cathode is heated by means of alternating-current each cathode portion will be heated by a thermionic bombardment and also by heat radiated from the adjacent portion or portions of the cathode. In the case where one of the cathode portions surrounds the other and the device is heated by direct-current as will be hereinafter described the outer portion is heated by electron bombardment from the inner portion and the inner portion is heated by'means of heat radiated from the outer portion and the operative temperature in this manner maintained.

The. cathode portions are preferably connected to a source of elect'romotive force external to the device or alternatively the the electromotive force may be induced across them, say by connecting them We coil winding internal to the device; in the latter case the heating current must be alternating.

It is desirable that the electron discharge between the cathode portions be confined to the interelectrode space and this condition may be attained by the use of shields or alternatively the activity for emission of electrons at the ends ofthe cathode portions 7 scribed. In an application of this method providing a combination of these means for reducing the temperature of the cathode edges, the edges of the cathode are made thinner than the centre portions thereof so that not only is the heat conducted from the gap of the cathode increased but the gap between the cathode portions is also increased or it may be provided that the edges have a thickened flange as will be hereinafter described.

If it is arranged that the space current flowing between the cathode portions is limited by the space charge then the cathode is stable in operation. Generally the result is such that below a certain temperature the cathode temperature will not be maintained. Thus there is in most cases a minimum tcmperature at which the cathode will operate. This minimum temperature depends on the material of which the cathode is constructed, the ratio of the surface area of the cathode to the area between the cathode portions and also on the length across the gap between the portions. When the gap is large the minimum operating temperature is lower than when the gap is small.

As has been pointed out the cathode is in itself stable in operation when the heating space current is limited by the space charge existing between the cathode portions and not by the emission current thereof. However, with some types of cathodes it is possible to arrange them so that they are stable in operation over a small temperature range even when the heating current is limited by the emission of the cathode portions.

When the heating current is limited by the emission from the cathode portions the device may be made stable by inserting suflicient resistance in series with the gap between the cathode portions so that the heating current must pass therethrough. This -'a view of a detail.

arrangement tends to produce local overheating of theicathode unless the cathode portions have a considerable cross-sectional area.

To enable the invention to be clearly understood it will now be described wit reference to the accompanying drawings in which Fig. 1 is a diagrammatic sectional view of a vacuum electric tube in accordance with the invention. diagram of suitable supply circuits for the tube. Fi 3 is a View of one arrangement of the tu e constructed in accordance with the invention and Figs. 4 and 5 are views of modified arrangements thereof. Fig. 6 is Figs. 7, 8 and 9 are views of further modified arrangements of the tube. Figs. 10, 11 and 12 are sectional views of details and Fig. 13 is a view of a still further arrangement of the tube.

In the drawings like reference numerals are applied to similar parts where possible.

,Referring to Fig. 1 which illustrates a rectifying valve, 1 is the cathode and 2 the anode, these electrodes being either contained in an envelope of suitable material or the anode 2 itself constituting a part of the wall of the envelope. The cathode 1 comprises two portions 3 and 4 disposed adjacent one another and preferably so that the separation or gap between them is small. The cathode portions 3 and 4 are supported respectively by-stems 5 and 6. In operation an alternating electromotvie force is applied to the cathode portions 3 and 4 such that an alternating thermionic space current flows across the gap between the portions. In this way each cathode portion will be heated by thermionic bombardment and also by heat radiating from the adjacent portion.

As previously stated the cathode 1 may be brougl'lt in various ways up to the requisite temperature which will be maintained by the application of the alternating electromotive force. In Fig. 1 an auxiliary member such as a coil filament is provided and heated by the passage therethrough of electric current for the purpose of setting up a thermionic bombardment of the cathode 1 by electrons emitted from the auxiliary cathode 7 which is made negative with respect to the main cathode. In this way the main cathode is heated to the temperature at which it will maintain itself by the bombardment of its two portions 3 and i when the alternating clect-romotive force is applied thereto. When such temperature has been obtained and is being maintained the auxiliary cathode 7 may be rendered inoperative and a convenient automatic arrangement to this end is illustrated in Fig. 2. If desired the heating space current may be prevented from passing to the anode by arranging that the edges of the cathode portions have a reduced activity for emission of electrons as will be Fig. 2 is an electrical 7 hereinafter described with reference to the modification wherein one cathode portion surrounds the othen 1 Referring to Fig. 2, during the operation of the valve the heating. current is supplied to the stems 5 and 6- through a transformer 8. During the initial heating of the cathode 1 by the auxiliary cathode 7 the latter is supplied with current from a transformer 9 and is made negative with respect to the main cathode 1 or to one of the portions thereof, for example by means of a battery or other suitable source of potential 10. The heating curent is caused to fiow through the operating coil 11 of a switch 12 which is so arranged that when the heating current for the main cathode flows through said operating coil the circuit of the auxiliary cathode filament 7 is broken. In order toexpedite the heating of the main cathode it is arranged that in the initial stages much higher voltage is applied to the cathode portions than durin normal operation. To this end the secondary winding of the transformer 8 is provided with tappings 13 from a which a lower voltage may be obtained than from'the terminals 14 of the transformer. The switch 12 is provided with full and medium voltage contacts so arranged that during the initial heating full voltage is applied to the cathode portions 3 and 4 through choke coils 15 which are so constructed that when the actual heating current flows the reactive drop induced therein is equal to the excess voltage across the terminals 14 over that across the tappings 13. Normally the switch 12 occupies its illustrated lower position in which the auxiliary cathode 7 is connected to transformer 9 and the main cathode 1 is connected to the full Voltage terminals 14 of the transformer 8, When the heating current attains its rated value the switch 12 is operated to its upper position and disconnects the auxiliary cathode 7 from the transformer 9 whilst the main cathode portions 3 and 4 are switched over to the medium voltage tappings 13 of the transformer 8, 16 represents the elementary circuit of a rectifier of alternating current supplied by a generator and passing through a direct current load as indicated, thus showing one manner of using the tube in a circuit.

Referring to the construction of tube il-. lustrated in Fig. 3 the anode 2 comprises an open-ended cylinder whilst the cathode 1 consists of two half-cylinders 3 and 4 of semi-circular cross-section mounted with their plane faces adjacent one another, the half-cylinders 3 and 4 being mounted respectively on the supporting stems 5 and 6. A helical grid 17 supported on a stem 18 is shown but it will be under stood for rectifying purposes this grid may 4 be absent. The cathode portions 3 and 4 may be supplied with alternating-current to maintain the operative temperature and they may be initially heated by means of an auxiliary cathode 7 in the manner described with reference to Fig. 1.

Referring to the construction of tube illustratedin Fig. 4 the anode 2 is cylindrical as before and the cathode 1 comprises three portions 19, 20 and 21 which may be of the shape shown. These portions are mounted on stems 22 supported in insulating members 23. The plane faces of the portions are separated from one another by a relatively short distance. Three-phase alternatingcurrent is applied to the cathode portions 19, 20 and 21 for example from the secondary winding 24 of a transformer. In this case the heating current flows across all the gaps existing between the cathode portions which are thus maintained at the operative temperature. The cathode 1 may be heated initially by means of an auxiliary cathode in the manner hereinbefore described or the grid 25, which may be formed of a continuous helix of wire and provided with a terminal lead 26 at each end thereof as shown in Fig. 4, may be heated by the passage of an electric current therethrough and utilized as an auxiliary cathode or alternatively it may be utilized as an auxiliarymember heating the cathode by "radiation therefrom. i

In the modified construction of tube shown in Fig. 5 the cathode comprises two helical windings 27 and 28 mounted so that the convolutions of the respective windings lie between one another, that is to say, the helices interlock wit-h one another but do not touch so that there is a gap or space between the cathode portions. rent is applied between the cathode portions 27 and 28 in order to maintain the temperature thereof in the manner previously described. In order initially to bring the cathode to the operative temperature a current may be passed through one of the helical windings for example the winding 28 which is provided with a terminal lead 29 at each end and the winding 27 is provided with a lead 30 at one end, whilst the other end thereof may be anchored at 31. In the preferred form the helices 27, 28 are wound of wire of rectangular cross-section as illustrated in Fig. 6.

Referring now to Fig. 7 in the construction of tube therein illustrated the cathode 1 comprises two co-axial cylinders 3 and 4 and mounted within the inner cylinder 4 preferably co-axially is an auxiliary heating filament 7 in the form of a helix supported for exampleby means of a stem 32 and a co-axial stem 33. In operation the outer cylinder or portion 3 of thecathode is made positive with respect to the inner cylindrical portion 4 so that a thermionic space current A'n alternating-cur- :diation or by electron bombardment from the auxiliary filament or cathode 7.

In the construction of tube illustrated in Fig. 7 it is intended that ionization in the discharge between the cathode portions 3 and 4 shall be reduced and to this end the inter-electrode space between the two cathode portions 3 and 4 and between the outer cathode portion 3 and the anode 2 are made small. Shields 34 and 35 are provided to confine the discharge to the inter-electrode spaces.

In the arrangement illustrated in Fig. 8 the cathode portions comprise a cylindrical portion 3 and coaxially within it a helical filament 4 the convolutions of which are relatively closely spaced from one another. The helix 4 is provided with a lead 32 and is supported on a coaxial stem 33 constituting its other lead. During operation the helical portion 4 of the cathode is made negative with respect to the cylindrical portion 3 and the cathode is brought up to the operative temperature by the passage of a current through the helical portion 4 as in the arrangement described in connection with Fig. 7. This initial heating current is not passed during the normal operation of the tube. In this case also the inter-electrode spaces are made small and the discharge is confined to the inter-electrodespace. In the tube shown in Fig. 8 this is effected by reducing the activity for the admission of electrons of the edges of the cathode portions. This is effected in the case of the outer portion 3 of the cathode.

by making it of greater length than the inner portion 4 so that the edges of the portion 3 are heated to a less extent. The inner portion 4 of the cathode may be so arranged and supported on the members 32 and 33 that its ends are sufliciently cool that appreciable electron emission will not take place from these parts.

Referring now to Fig. 9 the tube therein illustrated is a rectifier in which the anode 2 and the two portions 3 and 4 of the oathode 1 consist of coaxial cylinders having a small gap between them. The cathode is raised to the operative temperature and is maintained at this temperature in the manner described in connection with Fig. 7 The The initial heating discharges between the electrodes in the valve are confined to their respective interelectrode spaces by reducing the activity for emission of the electrons at the edges of the cathode. This is efiected in the example shown by increasing the length of the gap at the edges of the two portions of the cathode as indicated at 43. -As previously explained the larger gap 43 results in the edges of the outer portion 3 of the cathode being less strongly heated. If necessary the edges of the cathode may be cooled by connecting them to a heat conducting body or by providing them with a large radiatin surface.

Figs. 10, 11 and 12 illustrate various modifications of the arrangement shown in Fig. 9 for ensuring that the edges of the cathode shall remain at a temperature at which no electrons will be emitted from these parts. In Fig. 10 the inner por tion.4 of the cathode is extended as indicated at 45 beyond the extremity of the outer portion 3 of the oathode and is also reduced in diameter as shown. Preferably, the reduction in diameter of the inside portion of the cathode commences at a point inside the end of the cathode portion 3 so that decreased heating space current is supplied to the end of the outer portion of the cathode and the inner portion is cooled by virtue of its large radiating surface. In Fig. 11 the inner portion 4 of the cathode is provided with an annular groove or restriction 46 near each of its extremities. In this way by reason of the increased gap due to the groove or restriction 46 the edges of both cathode portions may be made sufficiently cool toprevent the emission of electrons therefrom. In Fig. 12 the thickness of both the cathode portions is reduced at 47 so that the gap between the ends thereof is increased to prevent or reduce the flow of heating current between them. It will be understood that by reducing the temperature of the ends of the cathode in the manner described the discharge between the cathode portions may be prevented from passing into the mam discharge and in the case wherein the inter-electrode space between the outer cathode portion and the anode is made small the space current flowing in the valve may be confined to this inter-electrode space.

In the modified construction illustrated in Fig. 13 the cathode comprises three plane trically positive and adjacent one or more of the edges of the plates 48, 49 and 50. The main cathode plates may thus be heated by electron bomardment from the auxiliary cathode 51 in the manner previously herein described. In a modification of the above arrangement the cathode ma comprise four sheets namely two outer s eets which will be maintained,

at a potential difierent from that of two inner sheets during normal operation and the auxiliary cathode which is employed for initially heating the cathode may be disposed between the two inner sheets. The filament which is employed as the auxiliary cathode for initially heating the main cathode parts may be employed during the operation of the valve as'a shield for confining the discharge between the cathode portions to the inter-electrode space between them.

It will be understood that various modifications may be made in the construction and arrangement of the valves without departin from the scope of the invention.

I 013111138 my invention 1. A vacuum electric tube operating with a substantially ure electron discharge, comprising an ano e and a cooperating cathode consisting of two relatively spaced portions of substantially equal mass and dimensions of which one surrounds and is made elecwith respect to the other so that the surrounding portion is heated by electron bombardment from the other portions and the latter is heated to its operative temperature by heat radiation from the surrounding portion.

2. A vacuum electric tube op a substantially pure electron discharge, comprising an anode and a cooperatindg cathode consisting of two relatively space portions of which onesurrounds and is made electrically positive with respect to the other so that the surrounding portion is heated by electron bombardment from the other portion and the latter is heated to its operative temperature b heat radiation from the surrounding portion, the gap between the edges of said other portion and the surrounding portion being greater than that between the remainder thereof and the surrounding portion so that said edges are less active for the emission of electrons and the heating currenhthereby prevented from flowing to the anode. v

3. A vacuum electric tube 0 crating with a substantially pure electron discharge, comerating with prising an anode and a cooperating cathode consisting of two relatively spaced portions of which one surrounds and is made electrically positive with respect to the other so that the surrounding portion is heated by electron bombardment from the other portion and the latter is heated to its operative temperature by heat radiation from the surrounding portion, the edges of at least one of said cathode portions being of less thick ness than the remainder thereof so that the heating space current is prevented from flowing to the anode.

4. A vacuum electric tube operating with a substantially pure electron discharge, comprising an anode, a cooperating cathodeconsisting of relatively spaced portions having means for applying an electromotive force between them so as to set up a space current.

between said portions to maintain the cathode at its operative temperature during the normal operation of the tube, and means heating space current is prevented from flowing to the anode.

5. A vacuum electric tube operating with a substantially pure. electron discharge, comprising an anode and a cooperating cathode consisting of two relatively spaced portions of which one surrounds and is made electrically positive with respect to the other so that the surrounding ortion is heated by electron bombardment 20m the other portion and the latter is heated to its operative temperature by heat radiation from the surrounding portion during the normal operation of the tube, and means for preventing the heating space current from flowing to the anode.

6. A vacuum electric tube operating with a substantially ure electron discharge, comprising an ano e and a cooperating cathode consisting of relatively spaced portions of substantially equal mass and dimensions having means for applying an electro-motive force between them to set up a substantially pure electron discharge between said portions to maintain the cathode at its operative temperature during the normal operation of the tube, and means for initially raising at least one portion of the cathode substantially to its electron-emitting temperature.

In testimony whereof I have hereunto subscribed my name this 10th day of July, 1923. 

